1. Field of the Invention
This invention relates to a catalyst for combustion and a process for producing the same. Particularly it relates to a catalyst carrier which hardly causes a reduction in activity and retains a high specific surface area at high temperatures, and a process for producing the same.
2. Description of the Prior Art
In recent years, research and development of making combustors highly efficient and compact by applying the so-called catalytic combustion principle wherein combustion is promoted using a catalyst, to various combustors such as gas turbine, have been actively advanced in various fields. Usually it is necessary for the catalyst used in such combustors that its activity does not diminish at high temperatures of 1,000.degree. C. or higher.
Further, in various petrochemistry-related industries, too, there is a tendency to use such catalysts at higher temperatures for yield improvement or for producing new products; hence improvement in the heat resistance of the catalyst has been a serious problem for its development.
The factors governing the heat resistance of catalysts vary depending on the catalyst, but in the case of a catalyst having an active ingredient supported on a carrier, the heat stabilities of the carrier and the active ingredient govern the heat resistance. Particularly in the case of a catalyst used at temperatures exceeding 1,000.degree. C., it is indispensable for enhancing the heat resistance that the carrier hardly sinters and can retain a high specific surface area at high temperatures. Thus, various inventions directed to oxides which are stable at high temperatures, that is, carriers having a high melting point and a specific surface area have been made. For example, stabilized zirconia (ZrO.sub.2), composite oxides such as mullite (2Al.sub.2 O.sub.3.3SiO.sub.2), spinel (MgAl.sub.2 O.sub.4), La-containing .beta.-Al.sub.2 O.sub.3, etc. have been studied (Japanese patent application Ser. No. Sho 59-92866/1984).
However, it is the present state of the art that any of these carriers are difficult to regard as having those characteristics necessary to obtain a high performance catalyst. For example, zirconia, mullite, etc. have a tendency that when they are kept at high temperatures of 1,000.degree. C. or higher for a long time, their specific surface areas lower gradually. La-containing .beta.-Al.sub.2 O.sub.3 has superior properties of retaining a high specific surface area at high temperatures, but lanthanum compounds are expensive as raw materials, and moreover there is a problem that unless it is prepared from an aqueous solution of a lanthanum salt and an aluminum salt according to a complicated coprecipitation method, its characteristic cannot be fully exhibited.